Despite some disadvantages, Invar (an Fe-Ni alloy with 36% nickel content) remains in strong demand as a tool for making composite parts.
“Invar has been a real important arrow in our quiver,” said Martin McLaughlin, Director of Space Structures for Northrop Grumman Aerospace Systems. “On the other hand, it makes a very heavy, expensive tool that is difficult to move. Its large thermal mass to heat and cool means longer cycle times in the autoclave. The upside is they last forever; they are very durable.”
The pros of Invar outweigh the cons, at least in terms of Coast Composites’ business.
“Customers that want tools for production generally select Invar,” said Paul Walsh, company President. “We produce about 500 Invar tools a year, most of which tend to be larger, our specialty. We also do a few graphite tools, but most are Invar.”
Most use their Invar tools with automated tape lay-up (ATL). For automatic fiber placement (AFP) machines, weight is a bigger issue since an AFP must rotate the mandrel. For automated tape layers, size and weight are not as important since the tool is stationary. Users frequently employ ATL for parts with mild contours and large, flat surfaces such as wings.
“We made the wing tools for the [Airbus] A350 and the [Boeing] 787. We built much of the fuselage tooling for the A350 and the 787 as well as the wing-tool demonstrator for the C-Series from Bombardier,” Walsh said, noting that wing tools in Invar are large, measuring 20 ft at the root by 100 ft long. He also notes that they typically hold 0.020-in tolerances on these large structures.
“We made a single 75,000-lb tool for the JSF [Joint Strike Fighter] for the entire wing and center fuselage section that is used on an AFP machine,” explained Walsh. “It balances well enough to be rotated by a single person by hand.”
The process for building a metal tool starts with first fabricating an “egg crate” substructure. Over that, they typically bump form and weld 0.75-in Invar sheet stock. After heat-treating, a five-axis CNC gantry mill finish machines the part to final tolerance to a finished thickness of about 0.5 in. Skilled labor finishes the detail tool for use.
“Recently, we have taken 15% of the weight out of our Invar tools,” said Walsh, using careful engineering to produce thinner tools with a stiffer support egg crate, a trend he sees continuing.
He also noted that schedules are getting tighter, with aerospace product teams refining designs sometimes even while the tool is going through final fabrication.
“We are researching ways to make these tools faster, including investments in more capable machinery and lean manufacturing techniques,” said Walsh. “Our newest five-axis machines can cut metal at up to 2500 in/min. This has made a significant improvement in our ability to turn around a tool quickly. We also invested in a heat-treat oven which helps us maintain cycle time particularly on large tools where movement on city streets is difficult.”